Reconstructed Z-contrast image of a CdSe nanocrystal embedded in a polymer film revealing the polarity of the lattice and an asymmetric shape. While the Cd terminated end is flat, the Se terminated end is pointed, appearing to end in just a single atomic site. This is not expected on the basis of the Wulff construction for equilibrium shapes, and indicates that the shape can be controlled by choice of passivating ligand (in this case TOPO).

John Cooke, Oak Ridge National Laboratory

Research Objectives
This research involves the investigation of the atomic-scale structure and the electronic properties of interfaces, grain boundaries, and nanocrystals using ab initio density functional calculations. The computation is done in conjunction with Z-contrast scanning transmission electron microscopy measurements, which provide atomic-resolution imaging of the structures of interest, and electron energy loss spectroscopy (EELS) measurements of local electronic structure.

Computational Approach
We use the density-functional theory in the local-density approximation, either in the plane-wave pseudopotential implementation (VASP code) or in the full-potential linearized augmented plane waves implementation (WIEN code).

Accomplishments
Localized defects at the SiC-SiO₂ interface: Theoretical studies of the Si-SiO₂ interface showed that contrary to expectation, a flat interface was energetically preferred, and therefore the observed defect states were the result of suboxide intrusions through the growth process. We have found that Si-Si suboxide bonds induce gap states near the conduction-band edge, while carbon precipitates at the interface determine an increase in the density of states near the valence-band edge.

Origin of electrical barriers at grain boundaries in SrTiO₃: We have shown through a combination of Z-contrast imaging, EELS, and theory that non-stoichiometry is an intrinsic effect at grain boundary dislocation cores. Theory then examined model structures consistent with the experimental data and showed that non-stoichiometry leads to lower energy boundaries.

CdSe nanocrystals: We have started investigating small CdSe clusters (10-30 atoms) as intermediate steps in the formation of CdSe nanocrystals in the nanometer size regime. We have determined the geometry of these clusters in the presence of organic passivants. We have found that organic ligands favor specific atomic configurations and surface coverages, and may determine the overall shape of the nanocrystals. This we expect will explain why the crystals grow with the asymmetric structure observed in a Z-contrast image.

Significance
SiC is becoming the semiconductor of choice for high-voltage, high-power applications, but its technological impact is still limited, mainly because of the poor electronic and transport properties of the SiC-SiO₂ interface. A better understanding of extended and localized defects at the interface is necessary to overcome the technological difficulties.

Perovskite oxides are the basis for a number of new materials systems with unusual and potentially very useful properties, such as high-temperature superconductivity, giant magnetoresistance, and ferroelectricity. In these systems the grain boundaries are electrically active, which sometimes is a serious problem. We have demonstrated the origin of the electrical barriers in SrTiO₃ to be non-stoichiometry.

Semiconductor nanocrystals can now be grown by chemical synthesis methods with a relatively high degree of control over the size and shape distribution. However, a microscopic understanding of the growth process and of the equilibrium shape as a function of the chemical environment and the growth conditions is still missing.

Publications
R. Buczko, S. J. Pennycook, and S. T. Pantelides, "Bonding arrangements at the Si-SiO₂ and SiC-SiO₂ interfaces and a possible origin of their contrasting properties," Phys. Rev. Lett. 84, 943 (2000).

S. T. Pantelides, G. Duscher, M. di Ventra, R. Buczko, K. McDonald, M. B. Huang, R. A. Weller, I. Baumvol, F. C. Stedile, C. Radtke, et al., "Atomic-scale engineering of the SiC-SiO₂ interface," Materials Science Forum 338, 1133 (2000).

M. Kim, G. Duscher, N. D. Browning, K. Sohlberg, S. T. Pantelides, and S. J. Pennycook, "Nonstoichiometry and the electrical activity of grain boundaries in SrTiO₃," Phys. Rev. Lett. 86, 4056 (2001).